U.S. patent number 9,887,317 [Application Number 14/636,939] was granted by the patent office on 2018-02-06 for light-emitting device and manufacturing method thereof.
This patent grant is currently assigned to EPISTAR CORPORATION. The grantee listed for this patent is EPISTAR CORPORATION. Invention is credited to Chiao-Yun Chang, Heng Li, Tien-Chang Lu.
United States Patent |
9,887,317 |
Lu , et al. |
February 6, 2018 |
Light-emitting device and manufacturing method thereof
Abstract
A light-emitting device including a substrate; a first
conductivity semiconductor layer disposed on the substrate; a first
barrier disposed on the first conductivity semiconductor layer; a
well disposed on the first barrier and including a first region
having a first energy gap and a second region having a second
energy gap and closer to the semiconductor layer than the first
region; a second barrier disposed on the well; and a second
conductivity semiconductor layer disposed on the second barrier;
wherein the first energy gap decreases along a stacking direction
of the light-emitting device and has a first gradient, the second
energy gap increases along the stacking direction and has a second
gradient, and an absolute value of the first gradient is smaller
than an absolute value of the second gradient.
Inventors: |
Lu; Tien-Chang (Hsinchu,
TW), Chang; Chiao-Yun (Taipei, TW), Li;
Heng (Yilan County, TW) |
Applicant: |
Name |
City |
State |
Country |
Type |
EPISTAR CORPORATION |
Hsinchu |
N/A |
TW |
|
|
Assignee: |
EPISTAR CORPORATION (Hsinchu,
TW)
|
Family
ID: |
55403502 |
Appl.
No.: |
14/636,939 |
Filed: |
March 3, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20160064596 A1 |
Mar 3, 2016 |
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Foreign Application Priority Data
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Sep 3, 2014 [TW] |
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103130523 A |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
33/06 (20130101); H01L 33/007 (20130101); H01L
33/12 (20130101) |
Current International
Class: |
H01L
29/15 (20060101); H01L 33/00 (20100101); H01L
33/06 (20100101); H01L 33/12 (20100101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3279266 |
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Apr 2002 |
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JP |
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3338778 |
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Oct 2002 |
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JP |
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2013178425 |
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Dec 2013 |
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WO |
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Other References
Hongping Zhao, et al. "Approaches for high internal quantum
efficiency green InGaN light-emitting diodes with large overlap
quantum wells" accepted Jun. 22, 2011; published Jul. 1, 2011 (C)
2011 OSA Jul. 4, 2011 / vol. 19, No. S4 /Optics Express, Consists
of 17 pages. cited by applicant .
Hongping Zhao et al. "Growths of staggered InGaN quantum wells
light-emitting diodes emitting at 520-525 nm employing graded
growth-temperature profile" Applied Physics Letters 95, 061104
(2009), consists of 4 pages. cited by applicant .
R. Vaxenburg et al. "Suppression of Auger-stimulated efficiency
droop in nitride-based light emitting diodes" Applied Physics
Letters 102, 031120 (2013), consists of 6 pages. cited by applicant
.
Ronald A. Arif et al. "Polarization engineering via staggered InGaN
quantum wells for radiative efficiency enhancement of light
emitting diodes" Applied Physics Letter 91, 091110 (2007), consists
of 4 pages. cited by applicant .
Ronald A. Arif et al. "Spontaneous Emission and Characteristics of
Staggered InGaN Quantum-Well Light-Emitting Diodes" IEEE Journal of
Quantum Electronics, vol. 44, No. 6, Jun. 2008, consists of 8
pages. cited by applicant .
Hongping Zhao et al. "Design Analysis of Staggered InGaN Quantum
Wells Light-Emitting Diodes at 500-540 nm" IEEE Journal of Selected
Topics in Quantum Electronics, vol. 15, No. 4, Jul./Aug. 2009,
consists of 11 pages. cited by applicant .
Chiao-Yun Chang et al. High efficiency InGaN/GaN light emitting
diodes with asymmetric triangular multiple quantum wells Applied
Physics Letters 104, 091111 (2014); doi: 10-1063/1.4867023,
consists of 6 pages. cited by applicant.
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Primary Examiner: Erdem; Fazli
Assistant Examiner: Wilson; Scott R
Attorney, Agent or Firm: Patterson + Sheridan, LLP
Claims
What is claimed is:
1. A method of manufacturing a light-emitting device comprising
steps of: forming a first conductivity semiconductor layer on a
substrate; forming a first barrier on the first conductivity
semiconductor layer; forming a well on the first barrier, wherein
the step of forming the well further comprises: forming a first
region at a first operational temperature in a first interval; and
forming a second region at a second operational temperature in a
second interval, wherein the first interval is followed by the
second interval; forming a second barrier on the well; and forming
a second conductivity semiconductor layer on the second barrier,
wherein the steps of forming the well, the first barrier, and the
second barrier comprising introducing a gallium based gas, an
indium based gas, and a nitrogen based gas, and a flow rate of the
gallium based gas, a flow rate of the indium based gas, and a flow
rate of the nitrogen based gas are maintained at fixed values,
respectively; wherein an absolute value of a slope of the second
operational temperature versus time in the second interval is
different from an absolute value of a slope of the first
operational temperature versus time in the first interval.
2. The method of claim 1, further comprising: forming a buffer
layer between the substrate and the first conductivity
semiconductor layer; and forming a strain releasing stack between
the first conductivity semiconductor layer and the first
barrier.
3. The method of claim 1, wherein the first operational temperature
is decreased linearly or stepwise from a first predetermined value
to a second predetermined value in the first interval, the second
operational temperature is increased linearly or stepwise from the
second predetermined value to a third predetermined value in the
second interval, and the third predetermined value is greater than
the first predetermined value.
4. The method of claim 3, wherein the step of forming the well
further comprises forming a third region by introducing the gallium
based gas, the indium based gas, and the nitrogen based gas at a
third operational temperature in a third interval, the third
interval is between the first interval and the second interval, and
the third operational temperature maintains at the second
predetermined value.
5. The method of claim 4, wherein the step of forming the first
barrier further comprises introducing the gallium based gas and the
nitrogen based gas at the first predetermined value.
6. The method of claim 4, wherein the step of forming the second
barrier further comprises introducing the gallium based gas and the
nitrogen based gas at the third predetermined value.
7. The method of claim 4, wherein the first predetermined value is
between 870 degrees Celsius and 900 degrees Celsius and the second
predetermined value is between 750 degrees Celsius and 780 degrees
Celsius.
8. The method of claim 1, wherein the first interval is two to
three times of the second interval or the second interval is two to
three times of the first interval.
9. A method of manufacturing a light-emitting device comprising
steps of: forming a first conductivity semiconductor layer on a
substrate; forming a first barrier on the first conductivity
semiconductor layer; forming a well on the first barrier, wherein
the well comprises a first region and a second region on the first
region along a stacking direction and the steps comprise: forming
the first region by introducing a gallium based gas, an indium
based gas, and a nitrogen based gas at a first operational
temperature in a first interval, wherein flow rates of the gallium
based gas, the indium based gas, and the nitrogen based gas are
maintained at fixed values; and forming the second region by
introducing the gallium based gas, the indium based gas, and the
nitrogen based gas at a second operational temperature in a second
interval after forming the first region; forming a second barrier
on the well; and forming a second conductivity semiconductor layer
on the second barrier; wherein the first region has a first energy
gap and the second region has a second energy gap, the first energy
gap is decreased along the stacking direction and has a first
gradient, the second energy gap is increased along the stacking
direction and has a second gradient, and an absolute value of the
first gradient is smaller than an absolute value of the second
gradient; wherein an absolute value of a slope of the second
operational temperature versus time in the second interval is
different from an absolute value of a slope of the first
operational temperature versus time in the first interval.
10. The method of claim 9, further comprising: forming a buffer
layer between the substrate and the first conductivity
semiconductor layer; and forming a strain releasing stack between
the first conductivity semiconductor layer and the first
barrier.
11. The method of claim 9, wherein a flow rate of the gallium based
gas, a flow rate of the indium based gas, and a flow rate of the
nitrogen based gas are constants, respectively.
12. The method of claim 9, wherein the first operational
temperature is decreased linearly or stepwise from a first
predetermined value to a second predetermined value in the first
interval, the second operational temperature is increased linearly
or stepwise from the second predetermined value to a third
predetermined value in the second interval, and the third
predetermined value is greater than the first predetermined
value.
13. The method of claim 12, wherein the step of forming the well
further comprises forming a third region by introducing the gallium
based gas, the indium based gas, and the nitrogen based gas at a
third operational temperature in a third interval, the third
interval is between the first interval and the second interval, and
the third operational temperature maintains at the second
predetermined value.
14. The method of claim 13, wherein the step of forming the first
barrier further comprises introducing the gallium base gas and the
nitrogen based gas at the first predetermined value.
15. The method of claim 13, wherein the step of forming the second
barrier further comprises introducing the gallium based gas and the
nitrogen based gas at the third predetermined value.
16. The method of claim 13, wherein the first predetermined value
is between 870 degrees Celsius and 900 degrees Celsius and the
second predetermined value is between 750 degrees Celsius and 780
degrees Celsius.
17. The method of claim 9, wherein the first interval is two to
three times of the second interval.
Description
REFERENCE TO RELATED APPLICATION
This present application claims the right of priority based on TW
application Serial No. 103130523, filed on Sep. 3, 2014, and the
content of which is hereby incorporated by reference in its
entirety.
TECHNICAL FIELD
The disclosure is related to a light-emitting device, and more
particularly, a light-emitting device with a quantum well
structure.
DESCRIPTION OF THE RELATED ART
In comparison with conventional light sources, the light-emitting
diode with longer service life, smaller volume, lighter weight, and
higher efficiency is widely adopted in optical display devices,
traffic lights, information storage apparatuses, communication
apparatuses, lighting apparatuses, and medical appliances. A
light-emitting diode can be used solely or connected to other
devices for forming a light-emitting device. For example, a
light-emitting diode can be disposed on a substrate and then
connected to a side of a carrier or soldered/glued on a carrier for
forming a light-emitting device. Additionally, the carrier further
includes an electrode which is electrically connected to the
light-emitting device.
Generally, a light-emitting diode may include an n-type
semiconductor layer, an active layer, and a p-type semiconductor
layer. In order to enhance the light efficiency of light-emitting
devices, a multi-quantum well structure is formed in the active
layer. How to enhance the light efficiency by a quantum well
structure becomes a major topic for improving the performance of
light-emitting diodes.
SUMMARY OF THE DISCLOSURE
The disclosure is relative to a light-emitting device including a
substrate; a first conductivity semiconductor layer disposed on the
substrate; a first barrier disposed on the first conductivity
semiconductor layer; a well disposed on the first barrier and
including a first region having a first energy gap and a second
region having a second energy gap and closer to the semiconductor
layer than the first region; a second barrier disposed on the well;
and a second conductivity semiconductor layer disposed on the
second barrier; wherein the first energy gap decreases along a
stacking direction of the light-emitting device and has a first
gradient, the second energy gap increases along the stacking
direction and has a second gradient, and an absolute value of the
first gradient is smaller than an absolute value of the second
gradient.
The disclosure is also relative to a method of manufacturing a
light-emitting device including steps of: forming a first
conductivity semiconductor layer on a substrate; forming a first
barrier on the first conductivity semiconductor layer; forming a
well on the first barrier, wherein the step of forming the well
further includes: forming a first region by introducing a gallium
based gas, an indium based gas, and a nitrogen based gas at a first
operational temperature in a first interval; and forming a second
region by introducing the gallium based gas, the indium based gas,
and the nitrogen based gas at a second operational temperature in a
second interval, wherein the second interval is later than the
first interval; forming a second barrier on the well; and forming a
second conductivity semiconductor layer on the second barrier.
BRIEF DESCRIPTION OF THE DRAWING
The accompanying drawing is included to provide easy understanding
of the present application, and is incorporated herein and
constitutes a part of this specification. The drawing illustrates
the embodiment of the present application and, together with the
description, serves to illustrate the principles of the present
application.
FIG. 1A shows a cross section of a light-emitting device in
accordance with a first embodiment of the present application.
FIG. 1B shows a detailed view of FIG. 1A.
FIG. 1C show a detailed alignment view of FIG. 1B.
FIG. 2A shows flow rates as functions of time while forming a well
and a barrier in accordance with the first embodiment of the
present application.
FIG. 2B shows a diagram of the well and the barrier in accordance
with the first embodiment of the present application.
FIG. 2C shows the operational temperature as a function of time
while forming the well and the barrier in accordance with the first
embodiment of the present application.
FIG. 2D shows energy bands and structures of the well and the
barrier in accordance with the first embodiment of the present
application.
FIG. 3A shows flow rates as functions of time while forming a well
and a barrier in accordance with a second embodiment of the present
application.
FIG. 3B shows a diagram of the well and the barrier in accordance
with the second embodiment of the present application.
FIG. 3C shows the operational temperature as a function of time
while forming the well and the barrier in accordance with the
second embodiment of the present application.
FIG. 3D shows energy bands and structures of the well and the
barrier in accordance with the second embodiment of the present
application.
FIG. 4 shows energy bands of the wells and the barriers of
light-emitting devices in accordance with the first embodiment and
the second embodiment of the present application and the
conventional art.
FIG. 5 shows internal quantum efficiency as functions of power for
the light-emitting devices in accordance with the first embodiment
and the second embodiment of the present application and the
conventional art.
FIG. 6A shows output power as functions of current density for the
light-emitting devices in accordance with the first embodiment and
the second embodiment of the present application and the
conventional art.
FIG. 6B shows normalized efficiency as functions of current density
for the light-emitting devices in accordance with the first
embodiment and the second embodiment of the present application and
the conventional art.
FIG. 7A shows concentration of carriers as functions of position
and energy band as functions of position for the well and the
barrier for the light-emitting device in accordance with the
conventional art.
FIG. 7B shows energy bands of the well and the barrier and Femi
energies of electrons and holes for the light-emitting device in
accordance with the conventional art.
FIG. 8A shows concentration of carriers as functions of position
and energy band as functions of position for the well and the
barrier in accordance with the first embodiment of the present
application.
FIG. 8B shows energy bands of the well and the barrier and Femi
energies of electrons and holes in accordance with the first
embodiment of the present application.
FIG. 9A shows concentration of carriers as functions of position
and energy band as functions of position for the well and the
barrier in accordance with the second embodiment of the present
application.
FIG. 9B shows energy bands of the well and the barrier and Femi
energies of electrons and holes in accordance with the second
embodiment of the present application.
FIG. 10 shows a simulation of the recombination rate as functions
of position for the light-emitting devices in accordance with the
first embodiment and the second embodiment of the present
application and the conventional art.
FIG. 11 shows a simulation of the normalized efficiency as
functions of current density for the light-emitting devices in
accordance with the first embodiment of the present application and
the conventional art.
DETAILED DESCRIPTION OF THE EMBODIMENTS
To better and concisely explain the present application, the same
name or the same reference number given or appeared in different
paragraphs or figures along the specification should has the same
or equivalent meanings while it is once defined anywhere of the
present application.
The following shows the description of embodiments of the present
application in accordance with the drawing.
FIG. 1A shows a cross section of a light-emitting device in
accordance with an embodiment of the present application. A
light-emitting device 1 includes a substrate 10, a nucleation layer
20, a buffer layer 30, a first conductivity semiconductor layer 40,
a strain releasing stack 50, an active layer 60, a second
conductivity semiconductor layer 70, a first electrode 80, and a
second electrode 90. In the embodiment, the abovementioned layers
are epitaxialiy grown on the substrate 10 by approaches such as
metal organic chemical vapor deposition (MOCVD) or molecular-beam
epitaxy (MBE) and the growth direction is indicated by an arrow
C.sub.N. The substrate can be a single crystal substrate, an
electrically conductive substrate, or an insulating substrate. The
electrically conductive substrate can be a silicon substrate, a
gallium nitride substrate, or a silicon carbide substrate. The
insulating substrate can be a sapphire substrate. In the
embodiment, each layer is epitaxially grown on the C plane of the
sapphire substrate by MOCVD and the substrate 10 optionally has a
patterned surface by etching for enhancing light extraction
efficiency. Additionally, for epitaxially growing the layers,
trimethylgallium (TMGa), triethylgallium (TEGa), trimethylaluminum
(TMAl), and trimethylindium (TMIn) can be used as group IIIA
sources; ammonia (NH.sub.3) can be used as a group VA source;
silane (SiH.sub.4), and bis-cyclopentadienyl magnesium (Cp.sub.2Mg)
can be used as dopant sources.
In order to reduce lattice mismatch between the substrate 10 and
the first conductivity semiconductor layer 40, the nucleation layer
20 and the buffer layer 30 can be sequentially formed between the
substrate 10 and the first conductivity semiconductor layer 40. The
thicknesses of the nucleation layer 20 and the buffer layer 30 can
be tens of nanometers (for example, 30 nm) and several micrometers,
(for example, 3 .mu.m) respectively. Materials of the nucleation
layer and buffer layer can be group IIIA-VA materials including but
not limited to gallium nitride or aluminum nitride.
The first conductivity semiconductor layer 40, for example, an
n-type semiconductor layer, is formed on the substrate 10, the
nucleation layer 20, and the buffer layer 30. In the embodiment,
the thickness of the first conductivity semiconductor layer is
several micrometers (for example, 2.5 .mu.m) and the material of
the first conductivity semiconductor layer 40 can be gallium
nitride. A ratio of group VA source (for example, ammonia) to the
group IIIA source (for example, trimethylgallium) can be 1000 for
forming the first conductivity semiconductor layer 40.
Additionally, by introducing silane as a doping source, GaN layer
with silicon dopants can be formed and functions as the first
conductivity semiconductor layer 40. The material of the first
conductivity semiconductor layer 40 is not limited hereto and can
be other group IIIA-VA material.
Similarly, for reducing the lattice mismatch between the first
conductivity semiconductor layer 40 and the active layer 60 to
decrease the crystal defects, a strain releasing stack 50 can be
formed on the first conductivity semiconductor layer 40. The strain
releasing stack 50 can have a superlattice structure by alternately
stacking two kinds of semiconductor layers with different
materials. The two kinds of semiconductor layers can be an indium
gallium nitride layer (InGaN) and a gallium nitride layer (GaN),
and thicknesses of the indium gallium nitride layer and the gallium
nitride layer can be hundreds of nanometers (for example, 120 nm).
Otherwise, the strain releasing stack 50 can be multi-layers with
different materials and similar efficacy.
The active layer 60 is formed after the strain releasing stack 50
is formed. Please refer to FIGS. 1B and 1C. FIG. 1B shows a
detailed view of FIG. 1A and FIG. 1C show a detailed alignment view
of FIG. 1B. In the embodiment, the active layer 60 includes a
multi-quantum well structure but is not limited to it. In other
embodiment, the active layer can include a single quantum well
structure and is formed by alternately stacking a plurality of
wells 601 and barriers 603. In the embodiment, one barrier 603 is
firstly formed on the strain releasing stack 50, one well 601 is
formed on such barrier 603, and another barrier 603 and another
well 601 are alternately formed on such well 601 repeatedly while
the last one is barrier 603 or well 601. The steps of forming the
abovementioned multi-quantum well structure of the active layer 60
can include forming the well 601 first, then forming the barrier
603, and alternately forming the well 601 and the barrier 603
repeatedly. The thickness of each of the wells 601 is several
nanometers (for example, 2 nm.about.3 nm), and the well 601 can
include three regions designated as region I 6010, region II 6012,
and region III 6014. In the embodiment, the region I 6010 is closer
to the first conductivity semiconductor layer 40 and the strain
releasing stack 50. The region II 6012 is disposed between the
region I 6010 and region III 6014, and the region III 6014 is away
from the first conductivity semiconductor layer 40 and the strain
releasing stack 50. The material of the barrier 603 can include a
group IIIA-VA material, for example, gallium nitride or aluminum
nitride. The material of the well 601 can include a group IIIA-VA
material, for example, In.sub.xGa.sub.(1-x)N,
Al.sub.xGa.sub.(1-x)N, Al.sub.xIn.sub.yGa.sub.(1-x-y)N,
Al.sub.xIn.sub.1-xN or combinations thereof, wherein 0.ltoreq.x,
y<1. In the embodiment, the material of the barrier 603 is
gallium nitride and the material of the well 601 is gallium indium
nitride. A ratio of the group VA source (for example, ammonia) to
the group IIIA source (for example, trimethylindium) can be 18000
for forming the well 601 of the active layer 60; a ratio of the
group VA source (for example, ammonia) to the group IIIA source
(for example, triethylgallium) can be 2000 for forming the barrier
603 of the active layer 60, but the present application is not
limited hereto.
The second conductivity semiconductor layer 70 is formed on the
active layer 60. In the embodiment, the second conductivity
semiconductor layer 70 can be a p-type conductivity semiconductor
layer, for example, a gallium nitride layer doped with magnesium,
dopants, but the present application is not limited hereto. The
material of the second conductivity semiconductor layer 70 can be
other group IIIA-VA material. In the embodiment, a ratio of the
group VA source (for example, ammonia) to the group IIIA source
(for example, trimethylgallium) can be 5000 for forming the second
conductivity semiconductor layer 70 and bis-cyclopentadienyl
magnesium can be used as a magnesium dopant source. After the
second conductivity semiconductor layer 70 is formed, the first
electrode 80 and the second electrode 90 are manufactured by
processes, such as lithography, etching, and metal deposition for
completing the light-emitting device 1. The abovementioned first
conductivity semiconductor layer 40 and the second conductivity
semiconductor layer 70 can be single layer or multilayers.
Additionally, an undoped semiconductor layer can be disposed on the
first conductivity semiconductor layer 40 or the second
conductivity semiconductor layer 70.
Referring to FIGS. 2A to 2D for further understanding the formation
of the active layer 601. FIG. 2A shows flow rates as functions of
time while forming the well 601 and the barrier 603 in accordance
with the first embodiment of the present application. FIG. 2B shows
a diagram of the well 601 and the barrier 603 in accordance with
the first embodiment of the present application. FIG. 2C shows
operational temperature as a function of time while forming the
well 601 and the barrier 603 in accordance with the first
embodiment of the present application. FIG. 2D shows energy bands
and structures of the well 601 and the barrier 603 in accordance
with the first embodiment of the present application. As above
mentioned, the active layer 60 is formed by alternately stacking
the plurality of wells 601 and barriers 603. As shown in FIGS. 2A
to 2D, the well 601 is between two barriers 603. While forming the
barrier 603, a gallium based gas such as triethylgallium (TEGa), an
indium based gas such as trimethylindium (TMIn) and a nitrogen
based gas such as ammonia (NH.sub.3) are introduced. In the present
embodiment, a flow rate of the gallium based gas FR1, a flow rate
of the indium based gas FR2, and a flow rate of the nitrogen based
gas FR3 are constants. The operational temperature for forming the
barrier 603 can maintain at a first predetermined value T.sub.1,
for example, 870 degrees Celsius. A thickness of the barrier 603 is
several nanometers to tens of nanometers (e.g., 12 nm).
While forming the well 601, in an interval between t.sub.1 and
t.sub.2 (about 160 seconds), the flow rate of the gallium based gas
FR1, the flow rate of the indium gas FR2, and the flow rate of
nitrogen based gas FR3 can be maintained at fixed values,
respectively, and the operational temperature is decreased from the
first predetermined value T.sub.1 (e.g., 870 degrees Celsius) to a
second predetermined value T.sub.2, (e.g., 755 degrees Celsius) for
forming the region I 6010 of the well 601. The operational
temperature can be decreased linearly, stepwise or in other ways.
Generally speaking, while epitaxialiy growing layers by MOCVD, the
indium content of the layer is increased as the operational
temperature is decreased. In other words, the indium content of the
layer is decreased as the operational temperature is increased. By
the above-mentioned approaches which adjust the operational
temperature, the indium content of the region I 6010 is modulated
to be increased along the stacking direction (indicated by arrow
C.sub.N) of the light-emitting device 1. A composition of the
region I 6010 can range from GaN to In.sub.0.25Ga.sub.0.75N, but,
the present application is not limited hereto. The rise of the
indium content of the region I 6010 can be linear, stepwise or in
other ways.
After the region I 6010 is formed, the region II 6012 of the well
601 is formed in an interval between t.sub.2 and t.sub.3 (about 60
seconds) and the operational temperature is maintained at the
second predetermined value T.sub.2. Additionally, the flow rate of
the gallium based gas FR1, the flow rate of the indium based gas
FR2, and the flow rate of the nitrogen based gas FR3 are maintained
at predetermined values. In the interval between t.sub.2 and
t.sub.3, the operational temperature is maintained at the second
predetermined value T.sub.2 and thus the indium content of the
region II 6012 is substantially constant (a composition of the
region II can be maintained at In.sub.0.25Ga.sub.0.75N).
After the region I 6010 and the region 6012 are formed, the region
III 6014 of the well 601 is formed in an interval between t.sub.3
and t.sub.4. In the interval between t.sub.3 and t.sub.4, the flow
rate of the gallium based gas FR1, the indium based gas FR2, the
nitrogen based gas FR3 are maintained at the abovementioned values,
and the operational temperature is increased linearly/stepwise to a
third predetermined value T.sub.3 so that the indium content of the
region III 6014 is decreased along the stacking direction. The
indium content of the region III 6014 can be decreased linearly or
stepwise. Additionally, for the individual well 601, the region III
6014 is closer to the second conductivity semiconductor layer 70
than the region I 6010, and the region II 6012 is formed between
the region I 6010 and the region III 6014. In other words, the
region I 6010, the region II 6012, and the region III 6014 are
formed in sequence. In other embodiment, the sequence can be
changed.
After the well 601 is formed, another barrier 603 is formed thereon
the gallium based gas such as TEGa, the indium based gas such as
TMIn, and the nitrogen based gas such as NH.sub.3 are introduced at
the same flow rate used in forming the above-mentioned well 601 and
the barrier 603, and the operational temperature is maintained at
the third predetermined value T.sub.3.
Additionally, the region I 6010 has an energy gap EI (not shown in
figures). The energy gap EI is decreased linearly, stepwise or in
other ways along the stacking direction (indicted by the arrow CN)
of the light-emitting device 1 and has a first gradient. In the
embodiment, the first gradient .DELTA.EI/.DELTA.DI is defined as an
energy gap difference per unit thickness in the region I 6010 while
the thickness is defined along the stacking direction CN. In the
embodiment, indium gallium nitride (InxGa1-xN) functions as the
well. The energy gap EI is decreased (x becomes bigger) since the
operational temperature is decreased along the stacking direction
while forming the region I 6010 so that the indium content is
increased.
In other respect, the region II 6012 has an energy gap EII (not
shown in figures). Since the operational temperature is maintained
at the second predetermined value T2 while forming the region II
6012, the indium content of the region II 6012 is substantially
fixed and the energy gap EII can be regarded as a constant. In
other words, the energy gap EII is devoid of gradient variation
along the stacking direction.
The region III 6014 has an energy gap EIII (not shown in figures)
which can be increased linearly, stepwise or in other ways along
the stacking direction as above mentioned and has a second gradient
EIII/.DELTA.DIII defined as an energy gap difference per unit
thickness in the region III 6014. As shown in FIG. 2D, the energy
gap EI difference is smaller than the energy gap EIII difference
from magnitude point of view. In other words, an absolute value of
the first gradient |.DELTA.EI/.DELTA.DI| is smaller than an
absolute value of the second gradient
|.DELTA.EIII/.DELTA..DELTA.DIII|. It is because while forming the
region I 6010, the operational temperature is varied from the first
predetermined value T1 to the second predetermined T2 in a longer
interval between t1 and t2, for example, 160 seconds, so that the
indium content in region I 6010 correspondingly varies from a lower
fraction to a higher fraction in such longer interval and the
operational temperature varies from the second predetermined value
T2 to the third predetermined value T3 in a shorter interval
between t3 and t4, for example, 60 seconds, so that the indium
content in region III 6014 varies from a higher fraction to a lower
fraction in such short interval. Additionally, as shown in FIG. 2D,
an average of the energy gap EI and an average of the energy gap
EIII are greater than the energy gap EII and the energy gap of the
barrier 603 is greater than the energy gap of the well 601.
In the embodiment, although the indium content of each region is
adjusted by the operational temperature so that the energy gaps of
different regions of the well are varied, the present application
is not limited to adjust the operational temperature or the
aforementioned gases, and the adjusted content is not limited to
indium content. In other embodiment, metal content, for example,
aluminum content of the well can be adjusted in other ways so that
the energy gap is adjusted and the absolute value of the first
gradient |.DELTA.EI/.DELTA.DI| can be smaller or greater than
|.DELTA.EIII/.DELTA.DIII|. For example, a material of the barrier
can include aluminum nitride (AlN), a material of the well can
include aluminum gallium nitride (AlxGa(1-x)N;
0.ltoreq.x.ltoreq.1), and the introduced gas can include aluminum
based gas. Since the energy gap of aluminum nitride is about 6.1
eV, which is greater than that of gallium nitride 3.4 eV, in order
to make the energy gap EI of the region I decrease along the
stacking direction and make the energy gap EIII of the region III
increase along the stacking direction, aluminum content in the
region I can be decreased along the stacking direction and aluminum
content in region III can be increased along the stacking
direction.
As shown in FIGS. 3A to 3D, FIG. 3A shows flow rates as functions
of time while forming the well and the barrier in accordance with
the second embodiment of the present application, FIG. 3B shows a
diagram of the well and the barrier in accordance with the second
embodiment of the present application, FIG. 3C shows the
operational temperature as a function of time while forming the
well and the barrier in accordance with the second embodiment of
the present application, and FIG. 3D shows energy bands and
structures of the well and the barrier in accordance with the
second embodiment of the present application. The second embodiment
in FIG. 3A to FIG. 3D is similar to the first embodiment in FIG. 2A
to FIG. 2D. One difference is in the structure of the well of the
active layer. In the second embodiment, the active layer includes a
well 601' and at least two barriers 603'. Similarly, a region I
6010' of the well 601' is formed in the interval between t1 and t2
and a region II 6012' of the well 601' is formed in the interval
between t3 and t4. In the embodiment, a composition of the region I
6010' is ranged from GaN to In0.25Ga0.75N, a composition of the
region II is In0.25Ga0.75N, and a composition of the region III
6014' is ranged from In0.25GaN0.75N to GaN. Another difference
between the first embodiment and the second embodiment is the
interval between t1 and t2 shown in FIGS. 3A to 3D is shorter than
the interval between t3 and t4 and thus an indium content
difference per unit time in the interval between t1 and t2 is
greater than an indium content difference per unit time in the
interval between t3 and t4. Accordingly, as shown in FIG. 3D, an
energy gap EI' difference of the region 6010' per unit thickness
DI' is smaller than an energy gap EIII'' difference of the region
6014' per unit thickness DIII' from magnitude point of view. In
other words, an absolute value of a first gradient
|.DELTA.EI/.DELTA.DI| in the second embodiment is greater than an
absolute value of a second gradient in the second embodiment
|.DELTA.EIII/.DELTA.DIII|.
In the present application, the intervals between t.sub.1 and
t.sub.2, t.sub.2 and t.sub.3, and t.sub.3 and t.sub.4 are not
limited to 160 seconds, 60 seconds, and 60 seconds or 60 seconds,
60 seconds, and 160 seconds. In other embodiment, in order to vary
indium contents in different ways in different intervals, the
intervals can correspond to different durations. For example, the
interval between t.sub.1 and t.sub.2 can be 2 to 3 times of the
interval between t.sub.3 and t.sub.4, the interval between t.sub.1
and t.sub.2 is shorter than the interval between t.sub.3 and
t.sub.4, or the interval between t.sub.1 and t.sub.2 and the
interval between t.sub.2 and t.sub.3 are longer than the interval
between t.sub.1 and t.sub.2. But the present application is not
limited hereto. As long as the operational temperature difference
(absolute value) per unit time in the interval between t.sub.1 and
t.sub.2 is different from the operational temperature difference
(absolute value) per unit time in the interval between t.sub.3 and
t.sub.4, absolute values of the indium contents per unit thickness
(gradient) in the region I and region III are different from each
other. Additionally, the first predetermined value, the second
predetermined value, and the third predetermined are not limited to
870 degrees Celsius, 755 degrees Celsius, and 875 degrees Celsius.
The first predetermined value and the third predetermined value can
be greater than the second predetermined value. In other
embodiment, the first predetermined value and the third
predetermined can be about 900 degrees Celsius and the second
predetermined value can be smaller than 900 degrees Celsius.
Moreover, in other embodiment, the first predetermined can be
between 870 degrees Celsius and 900 degrees Celsius, the second
predetermined value can be between 750 degrees Celsius and 780
degrees Celsius, and the third predetermined value can be between
870 degrees Celsius and 900 degrees Celsius.
FIG. 4 shows energy bands of wells and barriers of light-emitting
devices in accordance with the first embodiment and the second
embodiment of the present application and the conventional art. In
FIG. 4, S represents the conventional light-emitting device, G
represents the light-emitting device of the first embodiment, and N
represents the light-emitting device of the second embodiment. In
FIG. 4, materials of a well S01 and a barrier S03 of the
conventional light-emitting device can include indium gallium
nitride (In.sub.0.25Ga.sub.0.75N) and gallium nitride GaN,
respectively. The energy gap of the well S01 is fixed and is not
varied with its thickness.
FIG. 5 shows internal quantum efficiency as functions of power for
the light-emitting devices in accordance with the first embodiment
and the second embodiment of the present application and the
conventional art. As shown in FIG. 5, S represents the conventional
light-emitting device in FIG. 4 and A represents both of the
light-emitting devices of the first embodiment and the second
embodiment. In FIG. 5, it shows the internal quantum efficiency of
the light-emitting devices of the first embodiment and the second
embodiment is higher than the internal quantum efficiency of the
light-emitting device of the conventional art at the same power
value.
Please refer to FIGS. 6A and 6B. FIG. 6A shows output power as
functions of current density for the light-emitting devices in
accordance with the first embodiment and the second embodiment of
the present application and the conventional art. FIG. 6B shows the
normalized efficiency as functions of current density for the
light-emitting devices in accordance with the first embodiment and
the second embodiment of the present application and the
conventional art. In FIGS. 6A and 6B, S represents the conventional
light-emitting device, G represents the light-emitting device of
the first embodiment, and N represents the light-emitting device of
second embodiment. In FIG. 6A, the light-emitting devices are
measured at room temperature; in FIG. 6B, the measured output power
value of each of the light-emitting devices at room temperature is
normalized by the measured output power value of each of the
light-emitting devices at low temperature so that a trend of the
efficiency of the light-emitting device increasing with the current
density is investigated. As shown in FIG. 6A, at the same voltage
and a current density of 69 A/cm.sup.2, output power values of the
light-emitting devices of the first embodiment, the second
embodiment, and the conventional art are 136.8 mW, 122.7 mW, and
110.1 mW, respectively. The output power values of the
light-emitting devices of the first embodiment and the second
embodiment are increased by 24.3% and 11.4%, respectively, compared
with the conventional light-emitting device. As shown In FIG. 6B,
at a current density of 69 A/cm.sup.2, the normalized efficiency
values of the light-emitting devices of the first embodiment and
the conventional art are 73% and 61%, respectively. That means an
efficiency declining rate of the light-emitting device of the
present application is lower than that of the conventional
light-emitting device as the current density is increased.
In the embodiment, the external quantum efficiency (EQE) at a
current density of 13.8 A/cm.sup.2 for the light-emitting devices
of the conventional art, the first embodiment, and the second
embodiment are approximately 59.6%, 68.3%, and 66.5%, respectively.
The output power values of the light-emitting device of the first
embodiment and the second embodiment are increased by 11.7% and
5.8%, respectively, compared with the light-emitting device of the
conventional art. As above mentioned, at a current density of 13.8
A/cm.sup.2 or 69 A/cm.sup.2, the output power and the efficiency of
the light-emitting devices of the first embodiment and the second
embodiment are higher than that of the light-emitting device of the
conventional art.
Please refer to FIGS. 7A, 8A and 9A. FIGS. 7A to 9A show
simulations of concentrations of carriers versus position and
energy bands of the well and the barrier versus position under
bias. FIG. 7A shows concentration of carriers as functions of
position and energy bands as functions of position for the well and
the barrier for the light-emitting device in accordance with the
conventional art. FIG. 8A shows concentration of carriers as
functions of position and energy bands as functions of position for
the well 601 and the barrier 603 in accordance with the first
embodiment of the present application. FIG. 9A shows concentration
of carriers as functions of position and energy bands as functions
of position for the well 601' and the barrier 603' in accordance
with the second embodiment of the present application. In FIGS. 7A,
8A, and 9A, the structures (i.e. barrier, well, region I, region
II, region III) are labeled. As shown in FIGS. 7A, 8A, and 9A,
higher concentration of carriers (electrons/holes) presents in the
well. In the light-emitting device of the first embodiment or the
second embodiment, a position of a peak value of the concentration
of electrons s is closer to a position of a peak value of the
concentration of holes, compared with the conventional
light-emitting device. It means that each wave function
distribution of the electrons and the holes of the first embodiment
and the second embodiment overlaps more than that of the
light-emitting device of the conventional art and the recombination
rates in the light-emitting devices of the first embodiment and the
second embodiment are greater than the recombination rate of the
conventional light-emitting device. As shown in FIG. 8A, the
variation of the energy gap in region I 6010 of the well is smaller
than the variation of the energy gap in the region III 6014. While
operating the light-emitting of the first embodiment, the electrons
move from the region I 6010 to the region III 6014 and the holes
move from the region III 6014 to the region I 6010. With the above
structure, the speed of the holes is increased and the movement of
electrons is restrained so as to increase the efficiency and
decrease electrons overflow.
FIGS. 7B, 8B and 9B show simulations energy bands of the well and
the barrier and Femi energies of electrons and holes under bias.
FIG. 7B shows energy bands of the well and the barrier and Femi
energies of electrons and holes for the light-emitting device in
accordance with the conventional art. FIG. 8B shows energy bands of
the well and the barrier and Femi energies of electrons and holes
in accordance with the first embodiment of the present application.
FIG. 9B shows energy bands of the well and the barrier and Femi
energies of electrons and holes in accordance with the second
embodiment of the present application. There are four lines shown
in the FIGS. 7B, 8B, and 9E. An upper line and an upper broken line
in figures represent a conduction band profile and Femi energy of
electrons, respectively; a lower line and a lower broken line in
figures represent a valance band profile and Femi energy of holes,
respectively. In comparison with FIG. 7B, the Femi energy of
electrons in FIG. 8B is away from the minimum (valley value) of the
conduction band. It means the probability that electrons present in
the well 601 is higher than the probability that electrons present
in the well S01. Additionally, the area (as slash lines shown)
defined by the Femi energy and the conduction band in FIG. 8B is
higher than that of in FIG. 7B. That represents the amount of
electrons in the well 601 is higher than that of in the well
S01.
Please refer to FIG. 10 shows a simulation of the recombination
rate as functions of position for the light-emitting devices in
accordance with the first embodiment of the present application and
the conventional art. S, G, and N represent a conventional
light-emitting device, the light-emitting devices of the first
embodiment and the second embodiment, respectively. As shown in
FIG. 10, recombination rates of the active layers of the
light-emitting devices of the first embodiment, the second
embodiment are greater than that of the conventional light-emitting
device.
Please refer to FIG. 11, FIG. 11 shows a simulation of the
normalized efficiency as functions of current density for the
light-emitting devices in accordance with the first embodiment of
the present application and the conventional art. S and G represent
a conventional light-emitting device without a polarization field
and the light-emitting device of the first embodiment without a
polarization field, respectively. S-P and G-P represent a
conventional light-emitting device with a polarization filed (0.7
Mvoltcm.sup.-1) and the light-emitting device of the first
embodiment of the present application with a polarization filed
(0.7 Mvoltcm.sup.-1) respectively. As shown in FIG. 11, regardless
of a polarization filed, a declining rate of the efficiency of the
light-emitting device of the first embodiment of the present
application is smaller than that of the conventional light-emitting
device.
The principle and the efficiency of the present application
illustrated by the embodiments above are not the limitation of the
present application. Any person having ordinary skill in the art
can modify or change the aforementioned embodiments. Therefore, the
protection range of the rights in the present application will be
listed as the following claims.
* * * * *